For many years there has been a need to encode documents with permanent, tamperproof, machine-readable digital data. For example, it is customary at ports of entry to check passport numbers against lists of stolen and revoked passports, a procedure which would be faster and more accurate if the numbers were machine readable and far more reliable if the machine-readable data were tamperproof and included information such as a code number or name which could not be visually detected. The rightful bearer would furnish the code when using the passport. Credit cards and cash cards involve essentially the same problem and additionally should be adaptable to the recording and reading of machine-readable data of a temporary nature concerning transactions in which the cards are used.
One solution to such need is disclosed in U.S. Pat. No. 3,566,356 (Holm et al.) wherein a document may include two superposed magnetizable layers, one of ordinary H.sub.c (300 oersteds) at the surface of the document and the other of relatively high H.sub.c (2000 oersteds). Although sophisticated techniques are required to selectively alter recorded data on such a document, the data can be altered and thus is not as tamperproof as may be required for certain uses.